1. Field of the Invention
The present invention refers to a method of bending sheet metal pieces to a predetermined bending angle by means of a bending apparatus having a computer control means and comprising a movable bending bar and a fixed bottom die having a die bottom member, the height position thereof being adjustable in accordance with the bending angle to be achieved.
It is known in the art that the bending angle of a sheet metal piece which is processed in a bending apparatus having a movable bending bar and a cooperating bending die can be theoretically approximately defined by the penetration depth of the bending bar into the bottom die, the latter one having a well defined die aperture width. However, practice has shown that the effective bending angle exhibits smaller or larger deviations from the above mentioned theoretical bending angle due to tolerances of the thickness and the material quality of the sheet metal piece to be bent.
2. Prior Art
A certain improvement has been made possible by the use of a bending apparatus in which the bending angle can be adjusted and changed easily and exactly. Preferably, such an apparatus comprises a bottom die having an upper longitudinal aperture directed towards the bending bar and a die bottom member which can be adjusted in height position. The bending angle is exactly defined by the height position of the die bottom member, and by re-adjusting said position different bending angles can be realized without the need of exchanging the bottom die.
The bottom die comprises a longitudinally running groove whose opening is directed towards the bending bar, and the bending angle is defined by the distance between the two fixed upper edges of the groove running parallel to each other and by the height position of the top surface of the movable die bottom member. However, if the bending operation is performed with a plurality of sheet metal pieces having equal thicknesses and substantially equal material qualities, it was observed that deviations of the resulting bending angle occur nevertheless. On the one hand, these deviations are caused by the fact that never the theoretical sharp edge develops in the sheet metal piece as is present at the bending bar but the bent edge is more or less rounded which greatly influences the final bending angle. On the other hand, a resilient spring back movement of the two legs of the bent sheet metal workpiece can be observed as soon as the bending force is released from the workpiece with the result that the actual bending angle does not coincide with the theoretically calculated value of the bending angle. The spring back rate also depends on the thickness of the sheet metal piece and the material quality thereof.
In practice, this means that two sheet metal pieces having equal nominal thickness and substantially equal material quality but originating from different manufacturers or from different production batches never show exactly the same bending angle after having been bent with the same bending apparatus and the same bottom die set-up because the behaviour of the sheet metal piece materials can be slightly different as far as the resulting rounding of the bent edge and as the spring back rate is concerned.
In order to improve the accuracy of a bending operation, a plate bending apparatus is disclosed in U.S. Pat. No. 4,552,002 which utilizes a bending punch and a bottom die into which the bending punch penetrates to a greater or lesser extent depending on the desired bending angle. Thereby, the magnitude and variation of the bending force required during the bending operation of the sheet metal plate is measured and utilized to determine the depth of punch penetration. This apparatus yields quite good results.